doc: introduce Split Device Model feature for ACRN

Introduce Split Device Model feature for ACRN. Now we only implement pass through
PCI(e) device of it.

Tracked-On: #4371
Signed-off-by: Li Fei1 <fei1.li@intel.com>

doc/developer-guides/hld/hld-emulated-devices.rst

@@ -21,3 +21,4 @@ documented in this section.
    RTC emulation in hypervisor <rtc-virt-hld>
    Hostbridge emulation <hostbridge-virt-hld>
    AT keyboard controller emulation <atkbdc-virt-hld>
+   Split Device Model <split-dm>

doc/developer-guides/hld/hv-dev-passthrough.rst

@@ -155,24 +155,10 @@ directly. EPT mapping is not built for these pages having MSI-X table.
 Device configuration emulation
 ******************************
 
-PCI configuration is based on access of port 0xCF8/CFC. ACRN
+PCI configuration space could been accessed by PCI-compatible Configuration Mechanism
-implements PCI configuration emulation to handle 0xCF8/CFC to control
+(IO port 0xCF8/CFC) and PCI Express Enhanced Configuration Access Mechanism (PCI MMCONFIG).
-PCI device through two paths: implemented in hypervisor or in Service VM device
+ACRN hypervisor will trap these PCI configuration space access and emulate it. You may refer
-model.
+to :ref:`split-device-model` for detail.
-
-- When configuration emulation is in the hypervisor, the interception of
-  0xCF8/CFC port and emulation of PCI configuration space access are
-  tricky and unclean. Therefore the final solution is to reuse the
-  PCI emulation infrastructure of Service VM device model. The hypervisor
-  routes the UOS 0xCF8/CFC access to device model, and keeps blind to the
-  physical PCI devices. Upon receiving UOS PCI configuration space access
-  request, device model needs to emulate some critical space, for instance,
-  BAR, MSI capability, and INTLINE/INTPIN.
-
-- For other access, device model
-  reads/writes physical configuration space on behalf of UOS. To do
-  this, device model is linked with lib pci access to access physical PCI
-  device.
 
 MSI-X table emulation
 *********************

doc/developer-guides/hld/split-dm.rst

@@ -0,0 +1,58 @@
+.. _split-device-model:
+
+Split Device Model
+==================
+
+We usually emulate device in Device Model. However, we need to emulate device in
+ACRN Hypervisor for some reasons. For example, post-launched RTVM needs to emulate
+pass through PCI(e) devices in ACRN Hypervisor in which case it could continue to
+running even if the Device Model is killed. In spite of this, the Device Model still
+should own the overall resouce management like memroy/MMIO space, interrupt pin etc.
+So there shall be one communication method provided by ACRN Hypervisor, which used
+to align the resource information for Device Model to ACRN Hypervisor emulated device.
+
+Let's still take the pass through PCI(e) device as an example:
+Before we split pass through PCI(e) device from Device Model to ACRN Hypervisor, the
+whole picture is like this:
+
+.. figure:: images/split-dm-image1.png
+   :align: center
+   :width: 900px
+   :name: split-dm-architecture-overview1
+
+   PCI Config space access in Service VM or Pre-launched VM
+
+.. figure:: images/split-dm-image2.png
+   :align: center
+   :width: 900px
+   :name: split-dm-architecture-overview2
+
+   PCI Config space access in Post-launched VM
+
+After we split pass through PCI(e) device from Device Model to ACRN Hypervisor, the
+whole picture is like this:
+
+.. figure:: images/split-dm-image3.png
+   :align: center
+   :width: 900px
+   :name: split-dm-architecture-overview3
+
+.. figure:: images/split-dm-image4.png
+   :align: center
+   :width: 900px
+   :name: split-dm-architecture-overview4
+
+   PCI Config space access in Post-launched VM
+
+Interfaces Design
+=================
+
+In order to achieve this, we add a new pair of hypercall to align the PCI(e) BAR
+and INTx information.
+
+.. doxygenfunction:: hcall_assign_pcidev
+   :project: Project ACRN
+
+.. doxygenfunction:: hcall_deassign_pcidev
+   :project: Project ACRN
+